Hi all, hope everyone will be fine. I have two questions to ask as follows:

1) I dont know why the abundance value vs time of my GC/MS has decreased about two-fold from the previous values as I dont made any changes in GC/MS conditions and standard and/or sample preparation. As for instance previously the abundance value was in something like 200,000 but for last three measurements it has been decreasing and in more recent case it is in range of few thousand to around 50,000. What could be the reason and what are the possible solution?

2) I am preparing standards for 16 USEPA priority PAHs in the range of 0-200 ng/mL (0, 10, 20, 50, 100, and 200 ng/mL), however, I am facing the problem regarding standard 10 ng/mL, it shows me the peak area response (PAR) for Dibanzo[a,h]anthracene and Benzo[g,h,i]perylene double to the PAR of these compounds in 20 ng/mL. The PAR for these compounds is almost equal to PAR in 50 ng/mL. The peak area response for all other PAHs is fine there is no problem only these two compounds are posing problem (I am using these PAHs in mixture of 16 provided by company, so there should be no problem with above said two compounds as these are also members of mixture of 16 PAHs ). I already have checked my preparation procedure and there is no problem with that, so could anyone please help to figure out this problem. Beside this the procedural blank (0 ng/mL) is showing no peak except phenanthrene-d10 and fluoranthene-d10 added as internal standard, and zero peak area response for all these 16 PAHs, it means there is no problem with solvent as well.

Looking for kind response.

Regards,

Waqar